SABR is primarily utilized for stage T1–T2, node-negative tumors.

The goal of the workup for early-stage lesions treated with SABR is thus to rule out locally advanced or metastatic disease. This staging approach therefore includes computed tomography (CT) scan of the chest and positron emission tomography (PET)/CT study. Magnetic resonance imaging (MRI) of the brain is also recommended for patients with T2 disease or higher. Mediastinal evaluation should be considered in all patients, particularly those with stage T2 disease or higher or in centrally located lesions.

Postoperative radiation therapy is indicated in the following circumstances:

Given these indications for PORT, the following studies should be utilized to aid in target utilization:

Immobilization – Various immobilization systems are available for SABR. Early SABR used body frame systems that were designed to allow the patient to be imaged in a different room than the treatment was taking place, analogous to the head frame used in SRS. These include the BodyFix^TM and Body Pro-Lock^TM systems which also provided abdominal compression devices. With the advent of in-room CT, modified upper body cradles could be used as final volumetric imaging was now being done in the treatment room. SABR immobilization devices typically provide greater support than those utilized in standard fractionation regimens. Figure 2 demonstrates an example of an upper body cradle utilized for SABR (Fig. 2a) vs. that in conventional fractionation (Fig. 2b).

Simulation – CT simulations are performed with a slice thickness of 2.5–3 mm. Four-dimensional (4D) CT scans are acquired, to account for internal motion. Intravenous contrast can be considered when necessary to differentiate tumor involvement from mediastinal structures such as the vasculature, and this should be done on a traditional fast helical CT immediately after the 4DCT without moving the patient. When the magnitude of respiratory motion is less than 1 cm and regular, patients are typically treated with a “free-breathing” approach, in which they breathe regularly during a treatment designed to cover the entire track of the tumor motion. If irregular or motion >1 cm, then respiratory management is considered, either breath hold at deep inspiration or at expiration or free-breathing respiratory gating in which radiation is delivered at specific periods of the breathing cycle. Any gating technique should include daily imaging that can verify the gating level each day. Both breath-hold and free-breathing gating techniques have been shown to be beneficial in reducing target volumes for tumors that have substantial motion (Muirhead et al. 2010; Underberg et al. 2005). Deep inspiration breath hold has been shown to reduce overall lung dose by expanding the healthy lung away from the tumor, but for patients to this, they need to be able to maintain the appropriate position in the respiratory cycle for at least 15 s, which is difficult for a significant percentage of patients with lung cancer. They also need to be able to reproduce the location of the tumor on successive breath holds as verified by repeated breath-hold CTs; this may be added by video feedback or an occlusion value system such as that found in the ABC device.

Daily Localization – Daily volumetric imaging is used during treatment (cone-beam CT scan, CT on rails). The daily imaging must be performed in a way consistent with the delivery method, e.g., free-breathing CBCT for free-breathing patients, breath-hold CT for breath-hold patients, or dynamically gated CT for free-breathing gated patients. If volumetric imaging is not available, then fiducial implants can be placed to provide more accurate localization, though this fiducial placement should be correlated with bony anatomy and with each other daily to assess for interfraction migration.

Immobilization – Conventional fractionation upper body cradles with arms over the head are utilized (Fig. 2b).